At present, propylene is mostly isolated from the product mixture formed in the steam cracking of light naphtha. Economic and other reasons make it desirable to have a more flexible raw material basis. An alternative to isolation of propylene from mixtures in which it is present is the dehydrogenation of propane.
As a non-oxidative route, propylene can be obtained by dehydrogenation of propane over noble metal catalysts such as Pt/Al2O3, Pt/Sn/Al2O3 or over noble metal-free catalysts such as Cr/Al2O3. The reaction is strongly endothermic and proceeds at a satisfactory rate only at a high temperature. This promotes secondary reactions, eg. degradation of the propane to form ethylene and methane; at the same time, ethylene is hydrogenated by the hydrogen liberated in the dehydrogenation. The selectivity of the reaction decreases greatly with increasing conversion because of the by-product-dependent competing reactions, which makes the industrial implementability of the process questionable. In addition, secondary reactions lead to carbon deposits on the catalysts used, which would have to be regenerated after relatively short periods of operation.
In a process which has achieved industrial maturity, the dehydrogenation is carried out at low pressure and relatively high temperature and the catalyst is continuously regenerated using atmospheric oxygen (Energy Prog. (1986), 6(3) 171-6 and Chem. Eng. Today, Copying Uncertainty, Aust. Chem. Eng. Conf. 11th (1983), 663-71). The process can be carried out using Pt/Al2O3 catalysts in a moving bed at 600-700° C. and a pressure of 2-5 bar.
The process described in WO 9523123 uses Cr/Al2O3 catalysts which are operated cyclically, ie. using a regenerative procedure. In this process, the propane is preheated using the waste heat liberated in the burning-off of the carbon. Pt/Sn/Al2O3 catalysts are known from Shiyou Huagong (1992), 21(8), 511-515. That reference also discloses that these catalysts can be doped with potassium or magnesium. Doping with tin is said to slow the deactivation, despite formation of carbon deposits (Stud. Surf. Sci. Catal. 1994, 88, 519-24).
Oxidic catalysts comprising redox-active elements which are not present in their lowest oxidation state are described in EP-A-403 462.
The dehydrogenation of propane using zeolites of the ZSM-5 type is likewise known. If these zeolites are doped with zinc, this influences the acid-base behavior of the zeolites: cracking reactions are said to be largely suppressed (J. Chin. Inst. Chem. Eng. (1990), 21(3), 167-72).
The processes which have become known have, in particular, the disadvantage that the selectivity decreases greatly with increasing conversion. In addition, the catalysts have to be regenerated frequently, which is extremely disadvantageous for an industrial process.